1. Field of the Invention
The present invention relates to semiconductor integrated circuits and to a method of supplying a clock signal to internal blocks provided in a semiconductor integrated circuit.
2. Description of the Related Art
Owing to micro-miniaturization of circuit elements and a reduction in power supply voltage that have accompanied advances in semiconductor processes in recent years, semiconductor integrated circuits (referred to as an “LSI” hereinafter) have grown larger in size and operate at higher speeds.
Further, in order to reduce power consumption, the general practice is to cut off the supply of voltage to the LSI when the LSI is not operating or to control the supply of a clock to each of the blocks within the LSI. For example, the specification of U.S. Pat. No. 6,076,171 discloses an arrangement in which the clock frequency is changed depending upon the state of the CPU load in order to realize a reduction in power consumption.
While LSIs are increasing in size and are made to operate at high speeds, it is also required that an LSI exhibit a high degree of precision in terms of power supply voltage owing to the use of lower power supply voltages. In LSIs thus far, no problems arise if it is confirmed that a fluctuation in power supply voltage supplied during the operation of the LSI is within the rated range of such voltage fluctuation. However, as the allowable range of fluctuation in supplied voltage narrows due to use of lower power supply voltages, there are instances where the former circuitry that supplies power to the LSI cannot respond, leading to a large fluctuation in voltage, in a case where the power consumed by the LSI changes significantly in accordance with a changeover in the operating state thereof. In conventional arrangements, therefore, one problem is that when there is a large change in LSI power consumption in accordance with a changeover in the operating state, the supplied voltage exceeds the rated range of power-supply voltage fluctuation of the LSI, thereby leading to an LSI malfunction.
As the scale of the LSI gates and the LSI operating frequency increase in the future, it is predicted that the above-mentioned problem will become more conspicuous.
It should be noted that the aforementioned changeover in the operating state includes an on/off changeover in terms of the supply of power supply voltage to the interior of an LSI, a changeover between a normal operation and a power saving mode, and a changeover between a shut-down operation and a power saving mode. The power saving mode includes operation of the circuit at a frequency lower than that of normal operation, and operation only of some functions.